Key Snaps Only in One Cylinder After Rekey Decision Tree

Why this matters

You rekeyed several cylinders keyed alike, and the customer reports the key snapped off in exactly one of them while the others work fine. A single-cylinder break is a gift diagnostically: it tells you the key is sound (the others prove it) and the fault lives in that one lock. But it also tells you that cylinder is binding hard enough to shear hardened key steel, which means a real mechanical defect is hiding there, and simply extracting the stub and handing back a fresh key invites a repeat break. The job is to find why that one cylinder loaded the key to failure, fix it, and prove the fix before you leave. Single-cylinder breaks after a rekey point to a small set of causes: a binding pin from a burr or wrong-length pin, a sprung or galled plug, a cracked or distorted housing, or a key that was fatigued and finally let go on a slightly stiffer lock.

Symptom presentation

The break occurs in one cylinder of a set that is otherwise smooth. The customer often describes increasing stiffness in that lock before the snap, or that it always "took a little extra wiggle." The broken stub is usually lodged at the point of maximum torque, near full rotation or just past the resting position. Other cylinders cut from the same key turn freely. If multiple cylinders bind, the key or the keying is suspect and this single-cylinder tree does not apply.

Quick checks

• Confirm the other keyed-alike cylinders operate freely with a spare key cut off the same original, not the customer's worn daily copy. If the siblings turn with light, even torque, the key and the bitting are sound and the fault is isolated to the one lock.
• Inspect the broken stub before extracting. A clean transverse snap perpendicular to the blade suggests a sudden hard stop (a binding pin or a mechanical block) that loaded the key past its yield in one turn; a fatigued, bent, or partially cracked break with a dull, progressive fracture face suggests a tired key that finally let go on a slightly stiff lock.
• After extracting the stub, try to turn the plug a few degrees with a follower or a turning tool, with no key. Free rotation with light tension means the plug is clean; stiffness, a hard catch, or a spot where it will not pass confirms a mechanical bind independent of the key.
• Note where in rotation it broke. A break at or near the resting position points at insertion-area pin binding (a chamber the plug fights as it starts to move); a break near full throw points at the cam, bolt, or a tailpiece load downstream of the plug.
• Check the keyway mouth and the broken blade for galling marks or bright wiping. Scoring on the blade flats shows where it dragged before it snapped, which often points to the binding chamber.

Isolation tree

Branch 1: After extraction, the plug binds or catches when turned with a follower. The defect is inside the cylinder. Pull the plug under a follower and inspect each chamber under magnification. Look for a brass burr at a chamber mouth left by a worn pinning tool or a chipped follower, a bottom pin one increment too long that sits the stack proud of the shear line, a galled or scored plug, or a cocked top pin that a weak or bent spring has bound in its chamber. Gauge every bottom pin against the bitting before you re-pin; the offending chamber is usually the one whose pin reads a step off the chart. Deburr the chamber mouths, correct the pin to gauged length, dress any galling on the plug, replace a suspect spring, and re-pin to the existing key. A burr or a long bottom pin is the usual culprit on a fresh rekey.

Branch 2: Plug turns freely after extraction, but the cam or bolt mechanism behind it binds. The load that broke the key came from downstream: a tailpiece misaligned with the cam, a bolt dragging into a misaligned strike, or a sticky deadlatch. Bench-test the bolt independent of the cylinder, correct the strike or the cam-to-tailpiece engagement, and re-test under key torque.

Branch 3: Everything turns freely after extraction and inspection finds nothing wrong. Re-examine the key that broke. A key cut on worn stock, nicked at a deep cut, or repeatedly flexed can fail on the one cylinder that happened to be marginally stiffest, even within tolerance. Cut a fresh key on good stock from the original, verify smooth operation in that cylinder, and discard the suspect copy.

Branch 4: The housing or plug shows a crack, a distorted keyway, or a plug that no longer rotates concentrically. The cylinder is damaged, possibly from a prior forced entry or a defective casting. Replace the cylinder; do not return a cracked housing to service.

Confirming diagnosis

You have the right cause when the corrected cylinder turns with the same light, even torque as its keyed-alike siblings, key-tested 15 to 20 cycles with a fresh key. The discriminating test for a binding pin is a follower check after re-pinning: the plug should rotate the full keyway with no catch and the key should withdraw and insert cleanly. For a downstream load, the bolt should throw fully with light key effort and no rise in resistance through the stroke. If you cannot make that one cylinder match the feel of the others, you have not found the cause yet.

Remediation

• Internal bind (Branch 1): deburr chambers, correct pin length to gauge, dress galling or replace the plug, re-pin keyed-alike, relube dry.
• Downstream load (Branch 2): align strike, correct cam-to-tailpiece engagement, free the deadlatch, re-test.
• Fatigued key (Branch 3): cut a fresh key on good stock from the original; instruct the customer to retire worn copies.
• Damaged housing (Branch 4): replace the cylinder, pin to the existing key system, verify.
• On any single-cylinder break in a keyed-alike set, re-verify the surviving cylinders so a near-failure does not become the next callback.

References

• BHMA/ANSI A156.5, Auxiliary Locks and Associated Products (cylinder operation and durability).
• Schlage Commercial Pinning Charts and Cylinder Service Reference (bottom-pin increments and plug fit).
• Kwikset Rekey Kit and SmartKey Service Documentation (pin length, burr, and lubrication guidance).
• Sargent Cylinder Service Manual (plug, cam, and tailpiece engagement specifications).